Several different processes can be used to grow epitaxial layers of compound semiconductors and other materials. Chemical vapor deposition (CVD) enjoys great commercial importance because it grows layers relatively quickly and involves relatively inexpensive growth apparatus compared to molecular beam epitaxy (MBE). In CVD, one or more chemical species, each bearing an element to be incorporated into the grown material, flow in a gaseous phase over a heated wafer. The chemical species react at the surface, and the growth elements recombine on the wafer in the desired composition. For the growth of silicon, silane (SiH.sub.4) is the silicon precursor, and other precursors can be used for dopants, for example, phosphine (PH.sub.3) and arsine (AsH.sub.3) for the n-type dopants P and As. Vapor phase epitaxy (VPE) is an alternate name for CVD, which emphasizes that the new material can be grown to be epitaxial with the underlying wafer.
The interest in high-speed electronics and opto-electronics has prompted much research and development in the epitaxial growth of compound semiconductors such as GaAs and InP. These compound materials involve some somewhat exotic materials for which simple gaseous metal-hydride chemical precursors are not readily available. Furthermore, many interesting compound-semiconductor devices require the growth of exceedingly thin layers or of very abrupt junctions, of the order of manometers to tens of nanometers. MBE can satisfy most of these requirements although it cannot be used for P because the many forms of P prevents accurate control of its vapor pressure. The technology for CVD is thus desirable both for InP and for commercial environments demanding high production rates.
In organo-metallic chemical vapor deposition (OMCVD), at least some of the chemical precursors have a more complicated, organic form. For instance, for the growth of GaAs, as illustrated in the schematic diagram in FIG. 1, trimethylgallium ((CH.sub.3).sub.3 Ga) and arsine flow from respective supply tanks 10 in amounts regulated by respective valves 12. Trimethylgallium is in fact a liquid, but its vapor is obtained by bubbling a gas such as H.sub.2 through it. The gas flows combine through an unillustrated manifold into a common inlet port 14 to a generally illustrated reactor cell 16, which contains a graphite susceptor 18 on which is placed a wafer 20. The reactor cell 16 includes a main body wall 22, usually made of fused quartz and usually having a rectangular cross-section of the order of 15 cm.sup.2. An RF coil 24 external to the cell 16 controllably heats the graphite susceptor 18 and hence the wafer 20.
The combined gases flow over the wafer surface, and the arsine reacts and the trimethylgallium cracks on the surface of the heated substrate 20 to provide the As and Ga, which combine at the substrate surface in the crystalline arrangement of the substrate 20. That is, they are epitaxially deposited. A pump 26 maintains the gas flow and exhausts the gas but keeps the interior of the cell 16 at a fixed pressure. For the growth of InP, trimethylindium (CH.sub.3).sub.3 In) and phosphine can be used as precursors. Small-scale OMCVD reactors and complete growth systems usable with the invention are commercially available from Thomas Swan & Co., Ltd. of the United Kingdom, Spire Corporation of Massachusetts, and Emcore Corporation of New Jersey, among others.
Although OMCVD has at least partially solved the problem of precursors, the problem of control of thickness has been solved only for large commercial OMCVD reactors. See, for example, "Aixtrons's Low-Pressure MOVPE: Concepts for Improved Quality and Safety," III-Vs Review, volume 4, number 2, 1991, pp. 1, 3, 32, and 33. The thickness-control problem remains unsolved for less expensive, smaller reactors, as has been discussed by Thrush et al. in "MOCVD grown InP/InGaAs structures for optical receivers," Journal of Crystal Growth, volume 93, 1988, pp. 870-876 and by van de Ven et al. in "Gas phase depletion and flow dynamics in horizontal MOVCD reactors," Journal of Crystal Growth, volume 76, 1986, pp. 352-372. For the deposition of very thin layers, the gas flow must be uniformly maintained over the area of the wafer 20.
The lateral flow has been improved in the prior art by a number of schemes. In a first approach illustrated in plan view in FIGS. 2 and 3, the relatively small cylindrical inlet port 14 is joined to the relatively large main body 22 of the cell 16 by a long taper section 30. The susceptor 18 is placed in a trough 32 so that the wafer surface is near the lower side of the interior of the taper section 30. Additionally, a wedge 34 may be placed in the taper section 30 to force additional flow to the sides. The deposition can be made more uniform in the direction of gas flow by tilting the susceptor 18 by about 7.degree. about an axis perpendicular to the gas flow and with its surface toward the gas source so as to counter depletion of the reactants. The tilting can be accomplished by placing a fused quartz block 36 between the head of the trough 32 and the susceptor 18 so as to present a smooth surface over which the gas flows and then manually pushing a shim 38 to elevate the susceptor 18 to the desired angle. Additional uniformity can be obtained by a gas rotation technique in which the wafer 20 rests on a thin graphite disc disposed within the recess of the susceptor 18. A gas is externally supplied through channels in the susceptor 18 connected to a series of angularly offset nozzles in the recess to thereby continuously rotate the wafer 20 during the OMCVD deposition. In an alternative approach illustrated in FIGS. 4 and 5, a shower head 40 mostly fills the cross-section of the main body 22 of the cell 16. The head 40 contains a square array of holes 42 through which the gas flows with a significant pressure drop. Thereby, the gas is spread across the cell cross-section. Related approaches include baffles set in the gas flow.
Although the uniformity has been improved by these techniques, it has remained insufficient for advanced devices.